Many modern electronic devices include image sensors which convert optical signals to electrical signals. One type of the image sensors commonly used in the electronic devices is a stacked complementary metal-oxide-semiconductor (CMOS) image sensor (CIS). Based on different light paths, the stacked CIS includes a backside illumination (BSI) image sensor and a frontside illumination (FSI) image sensor. Typically, the BSI image sensor is fabricated by bonding a carrier wafer with an active wafer. In order to conduct the carrier wafer and the active wafer, a conductive via is formed to transmit the electrical signals converted by light-sensitive pixel devices of the BSI image sensor.